In internal gear machining using a general helical broach, an inner side of a fixed workpiece is machined to form a helical gear while rotating a helical broach in a broaching machine. In this process, a force which causes the workpiece to rotate in the same direction as the rotation direction of the helical broach generates in the workpiece.
Therefore, if the workpiece is not fixed to the broaching machine, the workpiece moves (rotates) in a circumferential direction when the helical broach is pulled out through the inner side of the workpiece from an upper side to a lower side. If the workpiece rotates during gear machining, the workpiece is displaced from an original position for machining by the helical broach, and consequently, the internal gear of the workpiece loses shape, so that a desired machining accuracy cannot be obtained.
In order to solve such a problem, a fixing means for fixing a workpiece to the broaching machine has been used to prevent the workpiece from rotating during gear machining. For instance, as a fixing means for fixing a workpiece, there is a device which firmly fixes the workpiece from above or a lateral surface side by providing a spike-shaped rotation stopper on a lower surface side of the workpiece in the broaching machine.
However, there has been a problem that when the spike-shaped rotation stopper is used for a workpiece in gear machining, a machining trace such as a scratch remains on the lower surface of the workpiece. Further, when fixing the workpiece from the upper side or the lateral side, a force for preventing the rotation of the workpiece during machining needs to be individually adjusted depending on the kind of the workpiece because the force varies depending on the material and size of the workpiece.
Therefore, Japanese Patent No. 4678746, U.S. Pat. No. 6,527,484, and JP 2005-144639 A disclose an art in which a rotation stopper for firmly fixing the workpiece to the broaching machine becomes unnecessary by limiting a helix angle of a gullet or a spiral flute in a helical broach to a predetermined range to reduce the force (rotational force) causing the workpiece to rotate (generated in the workpiece) during internal gear machining by the helical broach.